WASHINGTON >> The Trump administration believes Russia has restarted very low-yield nuclear tests, officials said today in a finding that could be used to renew in earnest the arms race between Moscow and Washington.

But the significance of the statements by the director of the Defense Intelligence Agency and a senior National Security Council official was immediately debated by nuclear weapons experts.

Some experts said claims of low-yield tests would be nothing new. Intelligence officials and nuclear analysts in Washington have long raised the possibility of such violations going back nearly two decades, to when Russia ratified the Comprehensive Nuclear Test Ban Treaty in 2000.

Other nuclear weapons experts have argued that significant Russian cheating on the treaty is unlikely because the designs of the country’s nuclear warheads tend to be very robust. The small returns, they have said, would make the geopolitical costs of getting caught prohibitively high.

Lt. Gen. Robert P. Ashley Jr., the director of the Pentagon’s intelligence arm, said Russia appeared to have enhanced its ability to develop new nonstrategic nuclear weapons and to manage its existing stockpile by its testing capability.

“The United States believes Russia is probably not adhering to the nuclear testing moratorium in a manner consistent with the zero-yield standard,” Ashley said in remarks at the Hudson Institute.

In a question-and-answer session afterward, he appeared to soften that statement, saying only that Russia “has the capability” to conduct a test with a low nuclear yield.

But in a later panel, Tim Morrison, a senior director at the National Security Council, said Russia had already violated the test ban treaty.

“General Ashley was clear that we believe that Russia has taken actions to improve its nuclear weapons capability that run contrary to the scope of its obligations under the treaty,” Morrison said.

The government officials said Russia had been developing a range of new tactical nuclear weapons — including low-yield arms meant to be used on conventional battlefields — that the tests could help make more reliable.

Siegfried S. Hecker, a former director of the Los Alamos weapons laboratory in New Mexico, said any low-yield testing by Russia at Novaya Zemlya, an Arctic Ocean archipelago, would most likely relate to experiments to enhance the safety and reliability of Russia’s nuclear arsenal — not trying to develop new kinds of warheads.

© 2019 The New York Times Company